Project Summary Quiescence is a common character of many stem cells. Low metabolic activity in these cells may function to minimize the potential damaging effects of stress, minimize the number of cells needed for replenishment, and may occupy unique niches. These cells are found in many adult human tissues, even in the tissues of the central nervous system. These naturally occurring stem cells however, are rare and difficult to isolate so a mechanistic understanding of their integrated cellular activities is largely unknown. We recently learned that the primordial germ cells (PGC) of the sea urchin are quiescent. From a highly active early embryonic cell, they rapidly and dramatically change their metabolic pathways, decrease their protein synthesis, have low mRNA turnover and diminished transcription, and reduced mitochondrial activity. These cells are quiescent at a time when the remainder of the embryo multiplies to thousands of cells with many unique gene and protein expression patterns. Following the quiescent period, the PGCs rapidly return to normal cellular activity. Thus, the phenotype is prolonged, inclusive, highly penetrant, and transient. Resolution of the phenotype will require large numbers of naturally occurring, isolated PGCs, high throughput molecular manipulations, quantitative biochemical approaches, and excellent clarity for in vivo optics ? features ideally suited for sea urchin embryos. This application will interrogate the molecular mechanism of this transient quiescence and the dramatic phenotypic changes in the stem cells of the germ line. Stem cell quiescence is shared by many stem cell types, in and out of the germ line, but sparingly few models are present to explore such mechanisms in abundance, in naturally occurring cells, and in vivo. Overall the novelty and impact of this work includes: 1) germline stem cells using oxidative glycolysis but without proliferation ? distinct from the Warburg effect; 2) broad, quantitative quiescence characters; 3) integration of metabolism with molecular mechanisms for quiescence; 4) analysis of quiescence on developmental potential of the stem cell, and 5) the cells studied are primordial germ cells, a cell type found in all sexually reproducing animals. Coupled with a tractable model system for study, this proposal provides outstanding potential to add to an exciting field of both basic and clinical interest for generation of sperm and eggs in the adult.